Microseism Source Direction in Cascadia Using Cross-correlation and Array Coherence

Thursday, 18 December 2014
Zhao Chen, Peter Gerstoft, Peter D Bromirski and Ravi Menon, University of California San Diego, La Jolla, CA, United States
The ongoing Cascadia Initiative (CI) is providing an increasing quantity of ocean bottom seismometer (OBS) data. These data are used to investigate the microseism source directions and to determine the surface wave tomography of the Cascadia region. Spatial asymmetry in the amplitude of cross-correlations between receiver pairs is observed in both primary microseism and double-frequency (DF) microseism bands. This asymmetry indicates that there are preferential microseism propagation directions, either because of source location and/or upper oceanic crustal structure. Ambient noise group velocity tomography was determined from cross-correlation of DF microseisms between receiver pairs. A low-velocity region oriented roughly north-south was identified, apparently associated with a thick sediment zone off the continental shelf, reflecting the strong influence of ocean bottom sediment layer thickness on DF microseism propagation. The coherence of DF microseism signals between different stations was lowest in the low-velocity thick-sediment region, consistent with the thick sediment layer strongly affecting microseism signal propagation.